update gcm server

[eyolas]

Hello

gcm-server isn't up to date. The message bean has new property (dry run : https://code.google.com/p/gcm/source/browse/gcm-server/src/com/google/android/gcm/server/Message.java).

[fernandospr]

Please, can you update this code with the latest from https://github.com/google/gcm/tree/master/client-libraries/java/rest-client/src/com/google/android/gcm/server and upload to Maven Central?

It has fixed several issues, like for example, retries on several scenarios https://github.com/google/gcm/issues/9

Thanks.